The medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions such as pacemakers, defibrillators, neuro-stimulators and therapeutic substance delivery pumps. Medical devices can be configured to be surgically implanted or connected externally to the patient receiving treatment. Clinicians use medical devices alone or in combination with therapeutic substance therapies and surgery to treat patient medical conditions. For some medical conditions, medical devices provide the best and sometimes the only therapy to restore an individual to a more healthful condition and a fuller life. One type of medical device is an implantable neurological stimulation system that can be used to treat conditions such as pain, movement disorders, pelvic floor disorders, gastroparesis, and a wide variety of other medical conditions. The neurostimulation system typically includes a neurostimulator, a stimulation lead, and an extension such as shown in Medtronic, Inc. brochure “Implantable Neurostimulation System” (1998). More specifically, the neurostimulator system can be an Itrel II™ Model 7424 or an Itrel 3™ Model 7425 available from Medtronic, Inc. in Minneapolis, Minn. that can be used to treat conditions such as pain, movement disorders and pelvic floor disorders. The neurostimulator is typically connected to a stimulation lead that has one or more electrodes to deliver electrical stimulation to a specific location in the patient's body.
Current leads that incorporate straight, silver cored, bundled-stranded-wire (“bsw”) wire have the weakness that all axial loading is transmitted, through the wires, to the electrical connections at either end of the device. Historically, coiled conductor designs absorbed much of the loading in the stretching of the coil. Additionally, existing lead body configurations are of a constant stiffness, distal to proximal ends. This brings about the compromise to find a stiffness that is suitable for the most rigorous requirement for the application. An example of this is the desire to stiffen the proximal end of the lead body to allow insertion through seal rings and ball seal connectors, while at the same time providing a lead body stiffness consistent with currently established neurological lead standards. For the most part, these are different requirements. A variable stiffness is desirable.